Experimental evidence suggests that rheumatoid arthritis is a disease of localized immune complex deposition. A large portion of these complexes are IgG rheumatoid factor. Because patients with non-rheumatoid disease frequently have elevated latex fixation titers, the role of rheumatoid factor in the pathogenesis of rheumatoid arthritis has been discounted. However, the properties of IgG rheumatoid factor in rheumatoid and non-rheumatoid disease have not been systematically compared. Using a recently developed radio-immunoassay we now propose to compare in detail the immunochemical characteristics of IgG and IgM rheumatoid factors in the sera of patients with rheumatoid vasculitis, seropositive and seronegative rheumatoid arthritis; and subacute bacterial endocarditis. We shall focus on the ability of IgG rheumatoid factors to form intermediate and high molecular weight complexes, their capacity of self-associate, their specificity and polyclonality, and their ability to bind complement. In patients with rehumatoid vasculitis, we further intend to study the temporal relationship of IgG rheumatoid factor to levels of circulating immune complexes, to IgM rheumatoid factor, and to clinical disease activity. We hope by these studies to better clarify the role of IgG rheumatoid factor in the etiology of rheumatoid arthritis.